Harkness Test - Well there you go!

First off, you have to consider the fact that Lamias are a different culture than humans. It is very likely that Lamias have a lower age of maturity than ours. Especially when you remember that Lamia culture is a VERY sexual one as evidenced in Chapter 27.

But there's of course one huge twist that you've forgotten:

MIIA WAS NEVER ACTUALLY THERE AS A HIGH SCHOOL EXCHANGE! That was simply a cover-up. She was in fact there to find a communal mate for her village! Her entire mission revolved around sex, and was therefore most likely of-age when she was sent out.

Though, with the laws in place, it likely wouldn't matter anyway, seeing that human-creature relationships are illegal.

I rest my case

But here's the catch: If a creature is more intelligent than a human, won't it be the one to view fucking a human as bestiality?